This invention relates to a separator for a fuel cell, and more particularly to an improved construction and composition of a separator for the fuel cell and method for manufacturing the same.
It is well known that a fuel cell produces electrical energy from a reaction of an enriched or reformed fuel and oxygen. Hydrogen is typically used as the fuel and may be obtained by reforming a methanol-water mixture in a reformer that is comprised of a catalyst and a heater for vaporizing the unreformed fuel. Air is normally the source of oxygen for the fuel cell. Often times, a plurality of cell units are stacked in series so as to increase the output voltage generating capacity of the fuel cell.
One type of cell unit is comprised of an anode, a cathode, and an electrolyte matrix impregnated with an electrolyte such as phosphoric acid interposed between the anode and cathode.
Aqueous phosphoric acid is known to be an excellent electrolyte in that it is stable but has a low vapor pressure at temperatures of 200 degrees Celsius and is also a good conductor. Phosphoric acid also rejects carbon dioxide, and at temperatures of 200 degrees Celsius the anode is able to withstand carbon containing impurities such as carbon monoxide in the fuel gas.
Separators are used for separating the anode of one cell unit from the cathode of an adjacent cell unit and for electrically connecting the cell units in series. The separators typically include a plurality of fuel passages on the anode side and a plurality of air passages, perpendicular to the fuel passages, on the cathode side of the separator. These passages act to separate the inward flow of fuel and air from each other. Once inside the fuel cell, the fuel and air react to produce electrical energy through an exothermic electrochemical reaction. In operation of the fuel cell, a reformed hydrogen-rich gaseous fuel is fed through the fuel passages into the cell where it is oxidized, releasing electrons to the anode. Air is simultaneously delivered through the air passages and is reduced in the cell so as to consume electrons. As a result, the fuel cell stack generates a voltage.
To attain maximum efficiency and power output in a phosphoric acid fuel cell, each separator must be impermeable to the hydrogen fuel and air, be a good electrical conductor, and be able to withstand corrosion by concentrated phosphoric acid at temperatures of 200 degrees celsius and below. To achieve these objectives, it is desirable that the separator be made of an impermeable material. Such a separator can be formed from a pure block of carbon to achieve the desired impermeability; however, this method if very expensive. Another method is to form the separator by pressing and sintering a carbon powder substance. While this process is considerably less expensive than forming a separator from pure carbon block, it has a disadvantage of producing a separator with a porous structure which can be susceptible to permeation by the reformed hydrogen fuel and air. This decreases the efficiency and output of the fuel cell.
It is, therefore, a principal object of this invention to provide an improved fuel cell having a separator which is inexpensive to manufacture but is constructed so as to prevent the reformed fuel and air from permeating the separator.
It is a further object of this invention to provide a method for manufacturing a fuel cell separator which is impermeable to reformed fuel and air.